A system of a speaker driving circuit for reproducing a stereophonic audio signal outputted from a personal computer via USB interface is introduced in Japanese laid open patent No. 2001-148894.
This system includes a decoder 15 that generate an audio signal L, R by decoding an audio data outputted from a personal computer via USB interface circuit 14, a sound volume control circuit 16 that control a level of generated audio signal L, R. The sound volume control circuit 16 is equipped with an amplifier 17L that amplify an acoustic level of a left channel audio signal in non-inverted mode, amplifier 17R that amplify an acoustic level of a left channel signal in inverted mode. Also equipped is a terminal of a voice coil of the speaker 18L, 18R each connected to the output of the amplifier 17L, 17R, and the other terminal of the voice coil is connected to ground level (GND).
The USB interface circuit 14, decoder 15, sound volume control circuit 16, and amplifier 17L, 17R are designed to operate by a power supply voltage of 5V, 0.1 A supplied by the personal computer via USB interface. The impedance of the voice coil of the speaker 18L, 18R are 32 Ω and the efficiency of the speaker is as high as 90 dB/W, so sufficient volume of acoustic power reproduction can be possible.
However, according to the above-mentioned system, there exist some problems. Namely, when the amplifier 17L, 17R is supplied by a single 5V power supply, the output of these amplifier are biased by a constant DC voltage. Accordingly bias current flows to the GND via voice coil independently with or without an audio signal, therefore futile power consumption occurs.
To avoid this bias current problem, the speakers 18L, 18R should be connected to each amplifier via coupling condenser, but if the system is designed for audio signal of frequency range 20 Hz-20 KHz, the requisite capacity of the coupling condenser is almost 250 μF for the speaker with 32 Ω impedance. Consequently it is difficult to implement such a large capacitive condenser to a portable device with a headphone.